The production of polymers at higher solids levels is desirable to provide a more efficient polymerization process, thereby increasing throughput. Until now, polymerization of water-soluble synthetic polymers such as polyacrylamide and copolymers thereof at high solids levels has been limited because of the great exothermic heat generated during polymerization. Generally, for example, the monomer content of a solution polymerization is limited to about 30% for acrylamide, for example, because of the temperature rise.
To overcome this problem, several methods have been considered. Among those proposed are a pressure process, where the reaction proceeds adiabatically and steam generation is suppressed by applying external pressure, and an evaporative process, where heat generated by the reaction is disipated by evaporation of water from the gel. The pressure process requires exponentially increasing external pressure to suppress boiling as the solids level rises, where the evaporative process requires large surface area for the reactants (e.g. thin film or sprayed droplets).
Alternative approaches to this problem are taken in Sumitomo, Jap. Ref. 57/63,305 and Flesher, U.S. Pat. No. 4,585,843. The Sumitomo approach involves the extraction of heat from the system during polymerization through the use of multiple heat exchange surfaces within the reactor. These cooling plates are placed in the reactor to form cavities, or subdivided sections, having thicknesses of 2-100 mm. This method is not attractive due to the very poor heat transfer from gels and the tendancy for the gel to stick to the heat transfer surface. Further, the teaching of Sumitomo is not applicable to a continuous process.
The Flesher approach to high solids polymers employs a chemical heat extraction process having a salt hydrate (e.g. sodium sulfate) that undergoes an endothermic change to extract heat from the system during polymerization. The endothermic compound must be non-reactive with the monomer or monomers and resultant polymers, and in sufficient amount to counterbalance the major part of the exothermic heat of reaction. The disadvantage of this method is that the final product retains a large amount, i.e. 30-50%, of the salt.
The continuous production of high molecular weight water-soluble synthetic polymers in Landolt et al., U.S. Pat. No. 4,138,539, represents the present state of the art. This provides an improved process for preparing a water-soluble synthetic polymer in a readily dissolved powder form. The Landolt patent discloses polymerization of a monomer solution at approximately 10.degree. C. with the polymerized gel emerging at approximately 95.degree. C. by adjusting the residence time for polymerization to provide a proper combination of temperature and time to achieve the desired conversion and molecular weight. Usually a residence time of 30 to 120 minutes is considered best by Landolt. Landolt utilizes a 30 weight percent monomer solution and obtains a polymer gel generally containing 31 to 40 weight percent polymer concentration. The Landolt process is deficient however because it cannot be used to polymerize monomer solutions having higher values of monomer weight percent resulting in higher polymer concentration in the polymer gel. The heat of polymerization cannot be removed rapidly enough so lower solids solutions have to be used. In view of the limited success of high solids levels in current polymerization processes, it is, therefore, an object of the present invention to provide a process for efficient, high solids polymerization.
It is a further object of the invention to provide an efficient high solids polymerization process which can be run as a continuous as well as a batch process.
It has been discovered that higher solids levels during the exothermic polymerization of water soluble polymers can be achieved through the use of a cooled monomer solution of a syrup or a partially frozen slurry which is then initiated. In this process, a cooled syrup or slurry of monomer crystals is initiated and polymerized to produce a high solids gel. The resulting exotherm of polymerization is depressed by the sensible heat of the solution and/or the latent heat of fusion of the frozen part of the monomer slurry thus allowing higher solids monomer solution to be polymerized.